1. Field of the Invention
The present invention relates to a horizontal deflection circuit for performing bidirectional scanning with an electron beam in the horizontal direction of a display screen, and a bidirectional horizontal deflection apparatus including such a circuit.
2. Description of the Related Art
In the field of display devices such as a cathode ray tube (hereinafter simply as xe2x80x9cCRTxe2x80x9d), the use of a bidirectional horizontal deflection apparatus suitable for displaying high definition images has been suggested. The bidirectional horizontal deflection apparatus is used to perform bidirectional scanning with an electron beam using a horizontal deflection coil. In the bidirectional horizontal deflection apparatus, the horizontal deflection coil equivalently has an inductance component and a resistance component connected in series therewith, and therefore pixels which should be aligned vertically could be shifted in the horizontal direction between forward and retrace scanning.
FIGS. 11A and 11B are charts for use in illustration of change with time in the horizontal deflection current passed across the horizontal deflection coil. FIGS. 12A and 12B are views for use in illustration of the state of a display screen corresponding to the current waveform of the horizontal deflection current shown in FIGS. 11A and 11B. Note that in FIGS. 12A and 12B, the coordinate in the horizontal direction is referred to as xe2x80x9cx-coordinatexe2x80x9d, and the coordinate in the vertical direction is referred to as xe2x80x9cy-coordinatexe2x80x9d.
FIG. 11A shows an ideal current waveform for a horizontal deflection current. In the state in FIG. 11A, the length of a trace scanning period T1 with an electron beam coincides with the length of a retrace scanning period T2, and time points t1 and t2 where current is zero during the forward and retrace scanning correspond to the midpoints of the scanning periods T1 and T2, respectively. Therefore, as shown in FIG. 12A, pixels which should be aligned vertically are displayed on the same x-coordinate in froward and retrace scanning. In forward and retrace scanning, pixels e1 and e2 in the center of the horizontal direction of the screen in FIG. 12A for example are displayed at positions on the same x-coordinate.
FIG. 11B shows a current waveform having a distortion caused by the resistance component of the horizontal deflection coil. In the state shown in FIG. 11B, the lengths of the forward and retrace scanning periods T3 and T4 coincide, while the midpoints t4 and t6 of forward and retrace scanning periods T3 and T4 do not coincide with time points t3 and t5 where the horizontal deflection current is zero. As a result, as shown in FIG. 12B, pixels which should be aligned vertically are shifted in a zigzag manner in forward and retrace scanning, in other words xe2x80x9czigzag vertical line interferencexe2x80x9d is caused. In forward and retrace scanning, pixels e1 and e2 for example in the center in the horizontal direction of the screen in FIG. 12B are displayed at different x-coordinates.
FIG. 13 is an equivalent circuit diagram of a configuration to prevent the zigzag vertical line interference by reducing the distortion in the horizontal deflection current. The circuit is used for example in a bidirectional horizontal deflection system disclosed by Japanese Patent Laid-Open No. 7-203238. In FIG. 13, since a horizontal deflection coil 212 and a negative resistance 218 are connected in series with each other. The negative resistance 218 cancels the resistance component RH of the horizontal deflection coil 212.
FIG. 14 is a waveform chart showing the operation of the equivalent circuit in FIG. 13. As shown in FIG. 14 at (a), voltage V1 applied to an input terminal 214 has a square waveform. Since the inductance LH of the horizontal deflection coil 212 is large, the horizontal deflection current passed across an output terminal 216 has a triangular waveform. The waveform of voltage VR generated with the negative resistance 218 is 90xc2x0 out of phase from the triangular waveform shown in FIG. 14 at (b). The voltage generated with the resistance component RH is cancelled by the voltage VR generated with the negative resistance 218, so that the horizontal deflection current passed across the equivalent circuit in FIG. 13 appears to have no loss by the resistance and no distortion. However, if the distortion of the current passed across the horizontal deflection coil 212 by the resistance component RH is cancelled with the negative resistance 218, the power consumption increases.
In the horizontal deflection system, as means for preventing the zigzag vertical line interference with small power consumption, Japanese Patent Laid-Open No. 8-172543 discloses a reciprocating deflection type CRT display device. The CRT display device includes zigzag vertical line interference automatic cancellation means which modulates a reading clock signal used to read out data corresponding to each pixel, changes the amount of delay of a horizontal synchronizing signal used as a reference for the reading clock signal, and stabilizes the changing horizontal size to cancel the zigzag vertical line interference.
FIG. 15 is a waveform chart for use in illustration of the operation of the conventional zigzag vertical line interference automatic cancellation means. The voltage waveform 227 in FIG. 15 at (a) is the waveform of pulse voltage VP applied to pass horizontal deflection current across the horizontal deflection coil. The voltage waveform 226 is the waveform of a voltage pulse obtained by frequency-dividing the reading clock signal into xc2xd. The current waveform 228 in FIG. 15 at (c) is the waveform of a horizontal scanning position signal in proportion to the waveform of the horizontal deflection current.
The zigzag vertical line interference automatic cancellation means compares the values I(A1) and I(B1), and values I(A2) and I(B2) of the horizontal scanning position signal having one-to-one correspondence with horizontal coordinates on a reproduced image at the starting point A1 and the midpoint A2 of the first trace scanning and at the ending point B1 and the midpoint B2 of retrace scanning following the trace scanning. The conventional zigzag vertical line interference automatic cancellation means detects the horizontal size based on the difference between the values I(A1) and I(A3) of the horizontal scanning position signal at the starting point A1 and the ending point A3 of the trace scanning.
FIG. 16 is a diagram showing the configuration of the conventional zigzag vertical line interference automatic cancellation means. The conventional zigzag vertical line interference automatic cancellation means shown in FIG. 16 includes horizontal deflection current detection means 303, decoder means 304 for sampling pulse generation, sample-hold means 305 to 309, subtractors 310 to 312, a comparison amplifier 313, variable delay means 314, a gain control circuit 315, and a power supply voltage control circuit 316.
The horizontal deflection current detection means 303 includes for example a resistor or/and a transformer, and is connected in series to a deflection coil 301 and an S distortion correction capacitor 302.
An input terminal 326 is provided with a signal produced by frequency-dividing the horizontal synchronizing signal into xc2xd. The decoder means 304 for sampling pulse generation is provided with a signal indicating the timing of reading out a pixel from a counter 317 forming a reading phase locked loop for generating a reading clock signal. The decoder means 304 for sampling pulse generation decodes the signal indicating the timing and outputs sampling signals 318 to 322 to the sample-hold means 305 to 309 depending on the signal at the input terminal 326.
The sample-hold means 305 to 309 sample and hold a horizontal scanning position signal I output from the horizontal deflection current detection means 303 in the timings corresponding to the sampling signals 318 to 322, respectively applied from the decoder means 304 for sampling pulse generation which will be described. Thus, the sample-holdmeans 305 to 309 hold the values I(A1), I(B1), I(A2), I(B2), and I(A3) of the horizontal scanning position signal I sampled in the timings of reading the pixels at the starting point A in the trace scanning, the ending point B, in the retrace scanning, the midpoint A2 in the trace scanning, the midpoint B2 in the retrace scanning and the ending point A3 in the trace scanning, respectively.
The subtracter 310 subtracts the value I(B1) of the horizontal scanning position signal held by the sample-hold means 306 from the value I(A1) of the horizontal scanning position signal held by the sample-hold means 305, and outputs the result to the variable delay means 314. The variable delay means 314 delays the horizontal synchronizing signal supplied to a phase locked loop circuit for generating a reading clock signal based on the output signal of the subtracter 310. Thus, the zigzag vertical line interference automatic cancellation means in FIG. 16 automatically cancels zigzag vertical line interference at the left end of a reproduced image.
The subtracter 311 subtracts the value I(B2) of the horizontal scanning position signal held by the sample-hold means 308 from the value I(A2) of the horizontal scanning position signal held by the sample-hold means 307, and outputs the result to the gain adjusting circuit 315. The gain adjusting circuit changes voltage to be applied to the voltage controlled oscillator in the phase locked loop circuit generating the reading clock signal. Thus, the gain adjusting circuit 315 changes the frequency of the reading clock signal. The zigzag vertical line interference automatic cancellation means in FIG. 16 thus automatically cancels zigzag vertical line interference in the center of a reproduced image.
The subtracter 312 subtracts the value I(A3) of the horizontal scanning position signal held by the sample-hold means 309 from the value I(A1) of the horizontal scanning position signal held by the sample-hold means 305, and applies the result to the non-inverting input terminal of the comparison amplifier 313. The comparison amplifier 313 amplifies the difference between the output signal of the subtracter 312 and a reference value IR applied to an input terminal 390 and applies the result to the power supply voltage control circuit 316. The power supply voltage control circuit 316 increase/reduces the power supply voltage applied to the horizontal deflection coil based on changes in the control input (I(A1)xe2x88x92I(A3)xe2x88x92IR) in order to keep constant the horizontal size.
As described above, in the bidirectional horizontal deflection system shown in FIG. 13, the use of the negative resistance 218 allows the zigzag vertical line interference to be prevented. The zigzag vertical line interference cancellation means as shown in FIG. 16 modulates the reading clock signal to cancel the zigzag vertical line interference.
However, the bidirectional horizontal deflection apparatus is typically provided with an EW correction circuit to correct east-west pincushion distortion (EW: right-left distortion). The EW correction circuit modulates horizontal deflection current using a parabolic waveform changing in a parabolic form at the vertical scanning interval. In this case, the horizontal deflection circuit supplying the horizontal deflection current to the horizontal deflection coil is affected by the parabolic waveform by the EW correction circuit supplying the horizontal deflection current to the horizontal deflection coil. The horizontal deflection circuit is affected by the vertical deflection current which is large current. As a result, a forward video signal and a backward video signal are shifted from one another at the top and bottom of the screen.
It is an object of the present invention to provide a horizontal deflection circuit capable of canceling zigzag vertical line interference in any positions of the top, center, and bottom of a screen and a bidirectional horizontal deflection apparatus including such a circuit.
A horizontal deflection circuit according to one aspect of the present invention performs bidirectional scanning with an electron beam in the horizontal direction of a screen for displaying an image and includes a horizontal deflection coil, a first current supply circuit for supplying the horizontal deflection coil with first horizontal deflection current for forward horizontal deflection, a second current supply circuit for supplying the horizontal deflection coil with second horizontal deflection current for backward horizontal deflection, a first driving circuit for driving the first current supply circuit so that voltage generated by the first horizontal deflection current is in synchronization with a first timing signal indicating a trace scanning period, a second driving circuit for driving the second current supply circuit so that voltage generated by the second horizontal deflection current is in synchronization with a second timing signal indicating a retrace scanning period, a deflection current correction circuit for correcting the first horizontal deflection current and the second horizontal deflection current based on a first correction waveform periodically changing at a vertical scanning intervals, and a driving timing correction circuit for correcting the driving timing of the first current supply circuit by the first driving circuit and the driving timing of the second current supply circuit by the second driving circuit based on a second correction waveform periodically changing at the vertical scanning intervals corresponding to the first correction waveform so that the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
In the horizontal deflection circuit according to the present invention, the first horizontal deflection coil is supplied with the first horizontal deflection current for forward horizontal deflection by the first current supply circuit and the second horizontal deflection current for backward horizontal deflection by the second current supply circuit. In this case, the first current supply circuit is driven by the first driving circuit so that voltage generated by the first horizontal deflection current is in synchronization with a first timing signal indicating a trace scanning period, and the second current supply circuit is driven by the second driving circuit so that voltage generated by the second horizontal deflection current is in synchronization with a second timing signal indicating a retrace scanning period. The first horizontal deflection current and second horizontal deflection current are corrected by the deflection current correction circuit based on a first correction waveform periodically changing at vertical scanning intervals. Thus, the deflection distortion of an image displayed on the screen based on a video signal can be corrected.
Furthermore, the driving timings of the first and second current supply circuits by the first and second driving circuits are corrected by the driving timing correction circuit based on a second correction waveform periodically changing at the vertical scanning intervals corresponding to the first correction waveform. Thus, the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
Thus, the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen. Therefore, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The driving timing correction circuit may control horizontal deflection amplitudes by the first and second driving circuits at the vertical scanning intervals based on the second correction waveform.
In this case, the horizontal deflection amplitude is controlled at the vertical scanning intervals based on the second correction waveform, and therefore the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen.
The driving timing correction circuit may control the amplitude of the second correction waveform based on a first amplitude control signal.
In this case, the amplitude of the second correction waveform is controlled based on the first amplitude control signal, and therefore the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen.
The deflection current correction circuit may include an east-west pincushion distortion correction circuit for correcting east-west pincushion distortion.
In this case, east-west pincushion distortion in an image displayed on the screen is corrected by the east-west pincushion distortion correction circuit. A shift in the driving timing of the first and second current supply circuits by the effect of the first correction waveform is corrected based on the second correction waveform at the vertical scanning intervals. As a result, while the east-west pincushion distortion is corrected, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The first and second correction waveforms may periodically change in a parabolic form at the vertical scanning intervals.
In this case, east-west pincushion distortion in an image displayed on the screen is corrected by the distortion current correction circuit based on the first correction waveform periodically changing in a parabolic form at the vertical scanning intervals. A shift in the driving timing of the first and second current supply circuits by the effect of the first correction waveform periodically changing in a parabolic form is corrected based on the second correction waveform periodically changing in a parabolic form at the vertical scanning intervals. Therefore, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The first driving circuit may include a first phase comparator for outputting voltage based on the phase difference between the first timing signal and voltage generated by the first horizontal deflection current, a first low-pass filter for integrating the voltage output from the first phase comparator, a first adder for adding the output voltage of the first low-pass filter and the output voltage of the driving timing correction circuit and outputting the result, and a first control circuit responsive to a first trigger signal and the output signal of the first adder for turning on and off the first current supply circuit, and the second driving circuit may include a second phase comparator for outputting voltage based on the phase difference between the second timing signal and voltage generated by the second horizontal deflection current, a second low-pass filter for integrating the voltage output from the second phase comparator, a second adder for adding the output voltage of the second low-pass filter and the output voltage of the driving timing correction circuit and outputting the result, and a second control circuit responsive to a second trigger signal and the output signal of the second adder for turning on and off the second current supply circuit.
In this case, the first phase comparator, the first low-pass filter, the first adder, and the first control circuit drive the first current supply circuit in synchronization with the first timing signal. The second phase comparator, the second low-pass filter, the second adder and the second control circuit drive the second current supply circuit in synchronization with the second timing signal. Furthermore, the output voltage of the first low-pass filter and the output voltage of the driving timing correction circuit are added by the first adder, the output voltage of the second low-pass filter and the output voltage of the driving timing correction circuit are added by the second adder. Thus, the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
The driving timing correction circuit may correct the driving timing of the first current supply circuit by the first driving circuit and the driving timing of the second current supply circuit by the second driving circuit based on a third correction waveform periodically changing at the vertical scanning intervals based on the second correction waveform and corresponding to the waveform of an interference signal so that the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
In this case, the driving timings of the first and second current supply circuits by the first and second driving circuits are corrected by the driving timing correction circuit based on a third correction waveform periodically changing at the vertical scanning intervals corresponding to the waveform of an interference signal in addition to the second correction waveform. Thus, if an interference signal is received, the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen. As a result, while the effect of the interference signal can be eliminated, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The driving timing correction circuit may control horizontal deflection amplitudes by the first and second driving circuits at the vertical scanning intervals based on the waveform of the interference signal.
In this case, the horizontal deflection amplitude is controlled at the vertical scanning intervals based on the waveform of the interference signal, and therefore the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen.
The driving timing correction circuit may control the amplitude of the third correction waveform based on a second amplitude control signal. The interference signal may be derived from vertical deflection current.
In this case, the amplitude of the third correction waveform is controlled based on the second amplitude control signal, and therefore the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen.
The waveform of the interference signal and the third correction waveform may periodically change in a saw-tooth waveform at the vertical scanning intervals. The interference signal may be derived from the vertical deflection current.
When an interference signal changing in a saw-tooth waveform such as vertical deflection current is received at the vertical scanning intervals, a shift in the driving timings of the first and second current supply circuits by the effect of the interference signal is corrected based on the third correction waveform periodically changing in a saw-tooth waveform at the vertical scanning intervals. As a result, when such an interference signal changing in a saw-tooth waveform is received, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
A horizontal deflection circuit according to another aspect of the present invention performs bidirectional scanning with an electron beam in the horizontal direction of a screen to display an image and includes a horizontal deflection coil, a first current supply circuit for supplying the horizontal deflection coil with first horizontal deflection current for forward horizontal deflection, a second current supply circuit for supplying the horizontal deflection coil with second horizontal deflection current for backward horizontal deflection, a first driving circuit for driving the first current supply circuit so that voltage generated by the first horizontal deflection current is in synchronization with a first timing signal indicating a trace scanning period, a second driving circuit for driving the second current supply circuit so that voltage generated by the second horizontal deflection current is in synchronization with a second timing signal indicating a retrace scanning period, and a driving timing correction circuit for correcting the driving timing of the first current supply circuit by the first driving circuit and the driving timing of the second current supply circuit by the second driving circuit based on a correction waveform periodically changing at vertical scanning intervals corresponding to the waveform of an interference signal so that the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
In the horizontal deflection circuit according to the present invention, the horizontal deflection coil is provided with the first horizontal deflection current for forward horizontal deflection by the first current supply circuit and the second horizontal deflection current for backward horizontal deflection by the second current supply circuit. In this case, the first current supply circuit is driven by the first driving circuit so that voltage generated by the first horizontal deflection current is in synchronization with the first timing signal, and the second current supply circuit is driven by the second driving circuit so that voltage generated by the second horizontal deflection current is in synchronization with the second timing signal indicating a retrace scanning period.
When an interference signal is received, the driving timings of the first and second current supply circuits by the first and second driving circuits are corrected based on a correction waveform periodically changing at the vertical scanning intervals corresponding to the waveform of the interference signal. Thus, the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
In this manner, the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen. As a result, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The driving timing correction circuit may control horizontal deflection amplitudes by the first and second driving circuits at the vertical scanning intervals based on the waveform of the interference signal.
In this case, the horizontal deflection amplitude is controlled at the vertical scanning intervals based on the waveform of the interference signal, and therefore the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen.
The driving timing correction circuit may change the amplitude of the correction waveform based on an amplitude control signal.
In this case, the amplitude of the correction waveform is controlled based on the amplitude control signal, and therefore the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen.
The waveform of the interference signal and the correction waveform may periodically change in a saw-tooth waveform at the vertical scanning intervals.
If an interference signal changing in a saw-tooth waveform such as vertical deflection current is received at the vertical scanning intervals, a shift in the driving timings of the first and second current supply circuits by the effect of the interference signal is corrected based on the correction waveform periodically changing in a saw-tooth waveform at the vertical scanning intervals. If therefore such an interference signal changing in a saw-tooth waveform is received, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The first driving circuit may include a first phase comparator for outputting voltage based on the phase difference between the first timing signal and voltage generated by the first horizontal deflection current, a first low-pass filter for integrating the voltage output from the first phase comparator, a first adder for adding the output voltage of the first low-pass filter and the output voltage of the driving timing correction circuit and outputting the result, and a first control circuit responsive to a first trigger signal and the output signal of the first adder for turning on and off the first current supply circuit, and the second driving circuit may include a second phase comparator for outputting voltage based on the phase difference between the second timing signal and voltage generated by the second horizontal deflection current, a second low-pass filter for integrating the voltage output from the second phase comparator, a second adder for adding the output voltage of the second low-pass filter and the output voltage of the driving timing correction circuit, and a second control circuit responsive to a second trigger signal and the output signal of the second adder for turning on and off the second current supply circuit.
In this case, the first phase comparator, the first low-pass filter, the first adder, and the first control circuit drive the first current supply circuit in synchronization with the first timing signal. The second phase comparator, the second low-pass filter, the second adder, and the second control circuit drive the second current supply circuit in synchronization with the second timing signal. Furthermore, the first adder adds the output voltage of the first low-pass filter and the output voltage of the driving timing correction circuit, and the second adder adds the output voltage of the second low-pass filter and the output voltage of the driving timing correction circuit. Thus, the driving timings of the first and second current supply circuits are corrected so that the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
A bidirectional horizontal deflection apparatus according to yet another aspect of the present invention performs bidirectional scanning with an electron beam in the horizontal direction of a screen and includes a storing circuit for storing image information, a first clock generation circuit for generating a first clock signal to write image information corresponding to forward and retrace scanning to the storing circuit, a second clock generation circuit for generating a second clock signal to read out image information corresponding to forward and retrace scanning from the storing circuit, a horizontal deflection circuit for deflecting an electron beam based on image information read out from the storing circuit forward and backward in the horizontal direction and forming a scanning line on the screen, a detection circuit for detecting the timing of an electron beam being at a prescribed position substantially in the center in the horizontal direction of the screen and outputting a detection signal based on the result, and a phase control circuit controlling the phase of the first or second clock signal generated by the first or second clock generation circuit so that a prescribed number of image information pieces are displayed from one end to the other end of each scanning line in forward and retrace scanning and image information corresponding to the prescribed position is displayed in synchronization with the detection signal generated by the detection circuit, and the horizontal deflection circuit includes a horizontal deflection coil, a first current supply circuit for supplying the horizontal deflection coil with first horizontal deflection current for forward horizontal deflection, a second current supply circuit for supplying the horizontal deflection coil with second horizontal deflection current for backward horizontal deflection, a first driving circuit for driving the first current supply circuit so that voltage generated by the first horizontal deflection current is in synchronization with a first timing signal indicating a trace scanning period, a second driving circuit for driving the second current supply circuit so that voltage generated by the second horizontal deflection current is in synchronization with a second timing signal indicating a retrace scanning period, a deflection current correction circuit for correcting the first horizontal deflection current and the second horizontal deflection current based on a first correction waveform periodically changing at vertical scanning intervals, and a driving timing correction circuit for correcting the driving timing of the first current supply circuit by the first driving circuit and the driving timing of the second current supply circuit by the second driving circuit based on a second correction waveform periodically changing at the vertical scanning intervals corresponding to the first correction waveform so that the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
In the bidirectional horizontal deflection apparatus according to the present invention, image information is written in the storing circuit in response to the first clock signal generated by the first clock generation circuit, and image information is read out from the storing circuit in response to the second clock signal generated by the second clock generation circuit. An electron beam based on image information read out from the storing circuit is deflected forward and backward in the horizontal direction by the horizontal deflection circuit and a scanning line is formed on the screen. The timing of the electron beam being at a prescribed position substantially in the center in the horizontal direction of the screen is detected by the detection circuit, and a detection signal indicating the timing is generated. The phase of the first or second clock signal generated by the first or second clock generation circuit is controlled by the phase control circuit, and therefore a prescribed number of image information pieces are displayed from one end to the other end of each scanning line in forward and retrace scanning, and image information corresponding to the prescribed position is displayed in synchronization with the detection signal generated by the detection circuit.
Thus, the timing of the electron beam being at the prescribed position substantially in the center in the horizontal direction of the screen coincides with the timing of image information corresponding to the prescribed position being displayed, so that zigzag vertical line interference can be prevented substantially in the center in the horizontal direction of the screen. The zigzag vertical line interference can be prevented by providing the detection circuit for detecting the timing of the electron beam being at a prescribed position substantially in the center in the horizontal direction of the screen and the phase control circuit controlling the phase of the first or second clock signal, in other words, the interference can be prevented in a simple manner.
Since the horizontal deflection circuit is provided, the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen. As a result, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The deflection current correction circuit may include an east-west pincushion distortion correction circuit for correcting east-west pincushion distortion.
In this case, the east-west pincushion distortion correction circuit corrects east-west pincushion distortion in an image displayed on the screen. A shift in the driving timings of the first and second current supply circuits by the effect of the first correction waveform is corrected based on the second correction waveform at the vertical scanning intervals. As a result, while east-west pincushion distortion is corrected, zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
A bidirectional horizontal deflection apparatus according to a still further aspect of the present invention performs bidirectional scanning with an electron beam in the horizontal direction of a screen and includes a storing circuit for storing image information, a first clock generation circuit for generating a first clock signal to write image information corresponding to forward and retrace scanning to the storing circuit, a second clock generation circuit for generating a second clock signal to read out image information corresponding to forward and retrace scanning from the storing circuit, a horizontal deflection circuit for deflecting an electron beam based on image information read out from the storing circuit forward and backward in the horizontal direction and forming a scanning line on the screen, a detection circuit for detecting the timing of an electron beam being at a prescribed position substantially in the center in the horizontal direction of the screen and generating a detection signal based on the result, and a phase control circuit for controlling the phase of the first or second clock signal generated by the first or second clock generation circuit so that a prescribed number of image information pieces are displayed from one end to the other end of each scanning line in forward and retrace scanning, and image information corresponding to the prescribed position is displayed in synchronization with the detection signal generated by the detection circuit, the horizontal deflection circuit includes a horizontal deflection coil, a first current supply circuit for supplying the horizontal deflection coil with first horizontal deflection current for forward horizontal deflection, a second current supply circuit for supplying the horizontal deflection coil with second horizontal deflection current for backward horizontal deflection, a first driving circuit for driving the first current supply circuit so that voltage generated by the first horizontal deflection current is in synchronization with a first timing signal indicating a trace scanning period, a second driving circuit for driving the second current supply circuit so that voltage generated by the second horizontal deflection current is in synchronization with a second timing signal indicating a retrace scanning period, and a driving timing correction circuit for correcting the driving timing of the first current supply circuit by the first driving circuit and the driving timing of the second current supply circuit by the second driving circuit based on a correction waveform periodically changing at vertical scanning intervals corresponding to the waveform of an interference signal so that the horizontal positions of corresponding portions of forward and backward video signals coincide in each position in the vertical direction of the screen.
In the bidirectional horizontal deflection apparatus according to the present invention, image information is written into the storing circuit in response to the first clock signal generated by the first clock generation circuit, and image information is read out from the storing circuit in response to the second clock signal generated by the second clock generation circuit. An electron beam based on image information read out from the storing circuit is deflected forward and backward in the horizontal direction by the horizontal deflection circuit and a scanning line is formed on the screen. The timing of the electron beam being at a prescribed position substantially in the center in the horizontal direction of the screen is detected by the detection circuit, and a detection signal indicating the timing is generated. The phase of the first or second clock signal generated by the first or second clock generation circuit is controlled by the phase control circuit, and therefore a prescribed number of image information pieces are displayed from one end to the other end of each scanning line in forward and retrace scanning, and image information corresponding to the prescribed position is displayed in synchronization with the detection signal generated by the detection circuit.
Thus, the timing of the electron beam being at the prescribed position substantially in the center of the screen coincides with the timing of image information corresponding to the prescribed position being displayed, and therefore zigzag vertical line interference can be prevented substantially in the center in the horizontal direction of the screen. The zigzag vertical line interference can be prevented by providing the detection circuit for detecting the timing of the electron beam being at the prescribed position substantially in the center in the horizontal direction of the screen and the phase control circuit for controlling the phase of the first or second clock signal, in other words, the interference can be prevented in a simple manner.
The horizontal deflection circuit is provided and therefore the starting timing of forward and retrace scanning can always be kept constant at the top, center, and bottom of the screen. As a result, the zigzag vertical line interference can be cancelled in any positions of the top, center, and bottom of the screen.
The waveform of the interference signal and the correction waveform may periodically change in a saw-tooth waveform at the vertical scanning intervals. The interference signal may be derived from vertical deflection current.
If an interference signal periodically changing in a saw-tooth waveform at the vertical scanning intervals such as vertical deflection current is received, a shift in the driving timings of the first and second current supply circuits by the effect of the interference signal is corrected based on the correction waveform periodically changing in a saw-tooth waveform at the vertical scanning intervals. As a result, if an interference signal changing in a saw-tooth waveform is received, zigzag vertical line interference can be cancelled in any positions at the top, center, and bottom of the screen.
The forgoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.